It has been known for some time that corrected anisometropes who wear spectacle lenses of different power in front of the two eyes are able to generate saccades that differ in amplitude in the two eyes exactly as required by the differing magnifications of the spectacle lenses. A common assumption is that this ability results from the operation of a neural adaptive mechanism, which, over time, gradually adjusts the relative amplitudes of the saccades produced by the two eyes. We now report that when the scenes viewed by the two eyes suddenly differ in size, the two eyes produce saccades that immediately differ in amplitude without any prior period of adaptation. Two slide projectors and an arrangement of orthogonal polarizing filters were used to present overlapping stationary random dot patterns simultaneously yet separately to the two eyes. The right eye always saw the same pattern while the left eye saw a pattern that was initially identical (pretest) and later replaced by one that was 8% smaller (test). The positions of both sides were recorded with the electromagnetic search coil, and horizontal saccades were elicited by target spots projected onto the pattern through polarizing filters so as to be visible only to the right eye. Subjects were three humans and three rhesus monkeys. Immediately upon viewing the smaller pattern, the left eye produced horizontal saccades that were significantly smaller than those produced by the right eye. This indicates that the saccadic system has some ability to cope immediately with aniseikonia, the compensation being almost complete in some subjects. We suggest that the important cue in these experiments is horizontal disparity and that the saccadic system uses this to scale the relative amplitudes of the saccades produced by the two eyes.